1. Field of the Invention
This invention relates to moving coil measuring instruments.
More particularly the invention relates to such instruments wherein the moving coil is supported for rotation about an axis defined by a pair of spaced support members mounted on a frame member of the instrument.
2. Description of Related Art
One known moving coil measuring instrument of this kind includes a magnetic circuit having two pole pieces which define an annular gap across which a magnetic field extends, the annular gap being substantially coaxial with the axis of rotation of the coil and a portion of the coil being disposed in the gap. In operation of this known instrument the interaction between an electrical current carried by the portion of the coil disposed in the gap and the magnetic field extending across the gap causes rotation of the coil, the angle through which the coil rotates indicating the value of the current carried by the coil.
The ideal linear operation of this known instrument, i.e. that equal increments in the current carried by the coil produce corresponding equal angular movements of the coil over the whole range of movement of the coil, can be adversely affected by the following: the tolerances of the various components of the instrument; the two pole pieces not being precisely concentric; foreign matter between mating surfaces of the instrument; the path taken by the portion of the coil disposed in the annular gap when the coil rotates not being precisely concentric with the annular gap; an unsymmetrical coil; and the support of the coil not being precise.